Supershell
A superbubble or supershell is a cavity which is hundreds of light years across, and is filled with 106 K gas blown into the interstellar medium by multiple supernovae and stellar winds. The winds of newly born stars strip superbubbles of any dust or gas. The Solar System lies near the center of an old superbubble, known as the Local Bubble, whose boundaries can be traced by a sudden rise in dust extinction of stars at distances greater than a few hundred light years.
The most massive stars, with masses ranging from eight to roughly one hundred solar masses and spectral types of O and early B are usually found in groups called OB associations. Massive O stars have strong stellar winds, and all of these stars explode as supernovae at the end of their lives.
The strongest stellar winds release kinetic energy of 1051ergs (1044J) over the lifetime of a star, which is equivalent to a supernova explosion. These winds can form stellar wind bubbles dozens of light years across. Inside OB associations, the stars are close enough that their wind bubbles merge, forming a giant bubble called a superbubble. When stars die, supernova explosions, similarly, drive blast waves that can reach even larger sizes, with expansion velocities up to several hundred km s−1. Stars in OB associations are not gravitationally bound, but they drift apart at small speeds (of around 20 km s−1), and they exhaust their fuel rapidly (after a few millions of years). As a result, most of their supernova explosions occur within the cavity formed by the stellar wind bubbles. These explosions never form a visible supernova remnant, but instead expend their energy in the hot interior as sound waves. Both stellar winds and stellar explosions thus power the expansion of the superbubble in the interstellar medium.
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